The present invention relates to a fuel injection system for an afterburner of a turbojet engine. Afterburners, of course, are well known and typically comprise a passage located downstream of the turbine of the turbojet engine through which pass the exhaust gases from the engine. In order to provide additional thrust to the turbojet engine, fuel is injected into this exhaust gas stream and ignited.
It is also known to provide individual fuel injectors extending in a generally radial direction into the afterburner. These injectors typically comprise plain tubes with their radially innermost ends sealed and the sidewalls perforated to define orifices through which the fuel issues. The orifices, as exemplified in British Patent No. 587,083, face in an upstream direction into the direction of flow of the exhaust gases. The injection of pressurized fuel into the exhaust gas stream through such a very small diameter orifice results in a highly localized fuel jet which requires an anvil located upstream of the opening to cause the fuel to splatter or disperse so as to render the fuel mixture as uniform as possible.
Such fuel injectors, however, do not provide a long operational life due to the clogging of the fuel orifices due to coking of the fuel, especially when the afterburner is passing from the non-operational to the operational mode. Another drawback of the injectors is that they provide a highly heterogeneous atomization of the fuel mixture along the periphery of the exhaust gas stream, thereby creating zones of different temperatures downstream of the injector. These zones cause improper operation of the afterburner and should be avoided if at all possible.
The afterburner shown in the aforementioned British patent utilizes an annular, perforated grille as the anvil or splatterer. The use of this structure causes large wakes in the exhaust gas stream that contributes to the heterogeneity of the fuel/gas mixture.
A second type of after burner injector is described in U.S. Pat. No. 3,044,264 to Seaward et al which utilizes a swirl jet generated by means of a fuel injection nozzle from which the fuel is injected tangentially into the exhaust gas stream. This device also suffers from poor atomization, excessive heterogeneity of the fuel/gas mixture downstream of the injection device and coking of the fuel injection nozzle orifice.
A third type, as disclosed in French Patent No. 1,454,312, utilizes a combination of pressurized fuel injectors and catalytic ignitors with a mixture of compressed air from the turbine to create a homogeneous mixture. This design requires the presence of two types of components, namely injectors and ignitors, in the gas stream thus requiring the afterburner to be more complex and inherently costlier to manufacturer and to maintain.
In an attempt to overcome the drawbacks of the aforementioned prior art devices, afterburner systems having circular fuel-injection manifolds associated with burner rings have been utilized with some success. These systems, as illustrated in French Patent No. 2,097,587, satisfactorily provide a homogeneous fuel/gas mixture. However, the devices present a substantial obstacle in the gas flow stream and generate substantially large wakes which deleteriously affect the turbojet engine operation when the afterburner is inoperative. The circular fuel injection manifolds typically have only a single fuel intake line, or at the most two such intakes, such that the response time for the initiation of the afterburner is increased due to the time required for the fuel to pass through the length of the manifold.
In addition to the aforementioned difficulties posed by the afterburner function, more recent turbojet engines operate at a substantially higher temperatures in the exhaust gas flow due to the improved design of the combustion chambers and to the use of ceramics, composites, or the like for the turbine wheels and blades. These increased operating temperatures have led to afterburner systems having several, concentric fuel injection manifolds. This, quite obviously, increases the complexity of the afterburner fuel injection system and typically requires a complex suspension system for the manifolds. The fuel intake tube must be cushioned relative to the wall of the gas passage, which may result in leakages at the wall where the intake tube passes through it. These multiple fuel injection manifolds further increase the time for the fuel to pass completely through them, thereby compounding the coking problems when changing the operational mode of the afterburner. This problem has only been incompletely remedied by a purge box which may be associated with the intake tube.